1. Field of the Invention
This invention is related to an optical system for use with short wavelength radiation in photolithography equipment.
2. Background of the Invention
Photolithography is a well-known manufacturing process used to create devices upon substrates. The process typically involves exposing a patterned mask to collimated radiation thereby producing patterned radiation, which is passed through an optical reduction system. The reduced patterned radiation or mask image is projected onto a substrate coated with photoresist. Radiation exposure changes the properties of the photoresist, allowing subsequent processing.
Exposure tools used in photolithography have two common methods of projecting a patterned mask onto a substrate: xe2x80x9cstep and repeatxe2x80x9d and xe2x80x9cstep and scan.xe2x80x9d The step and repeat method sequentially exposes portions of a substrate to a mask image. The step and repeat optical system has a projection field that is large enough to project at least one die image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
In contrast, the step and scan method scans the mask or reticle onto a wafer substrate over an annular field or a slit field that is the full height of one or more of the dies. Referring to FIG. 1, a ring field lithography system 100 for use in the step and scan method is shown. A moving mask 101 is illuminated by a radiation beam 103, which reflects off the mask 101 and is directed through a reduction ring field optical system 107. Within the optical system 107, the image is inverted and the arcuate shaped ring field 109 is projected onto a moving substrate 111. The arcuate shaped reduced image ring field 109 can only project a portion of the mask 101, thus the reduced image ring field 109 must scan the complete mask 101 onto the substrate 111. Because the mask 101 and substrate 111 move synchronously, a sharp image is scanned onto the substrate 111. The dimensions of the arcuate slit are typically described by a ring field radius and a ring field width.
These step and scan ring field systems have less distortion than step and repeat systems because it is easier to correct distortion over the narrow slit width. Referring to FIG. 2, an image is projected by the optical system onto to wafer through an arcuate ring field slit 201, which is geometrically described by a ring field radius 203, a ring field width 205, and a length 207. Ring field coverage is up to 180xc2x0 in azimuth 209.
As manufacturing methods improve, the minimum resolution dimension or critical dimension (CD) that can be achieved decreases, thereby allowing more electronic components or devices to be fabricated within a given area of a substrate. The number of devices that can be fabricated within an area of substrate is known as device density. With existing technology, 0.18 xcexcm resolution is possible using projection systems designed to operate at either 248 or 193 nanometers. One well-known means of improving the resolution dimension and increasing device density is to use shorter exposure wavelength radiation during photolithography processes. The relationship between resolution (R) and radiation wavelength is described by the formula: R=(K1xcex)/(NA), wherein R is the resolution dimension, K1 is a process dependent constant (typically 0.7), xcex is the wavelength of the radiation, and NA is the numerical aperture of the optical system at the wafer plane. Either reducing the operating wavelength or an increasing in the numerical aperture will improve the resolution of the system.
Improving the resolution by increasing the numerical aperture (NA) has several drawbacks. The most prevalent drawback is the concomitant loss of depth of focus. The depth of focus determines, in part, the process latitude or the available xe2x80x9cprocess windowxe2x80x9d. A reduced depth of focus limits the available process window. The relationship between NA and depth of focus (DOF) is described by the formula: DOF=(K2xcex)/NA2, wherein DOF is depth of focus, and K2 is a process dependent constant (typically close to unity). This simple relationship shows the inverse relationship between DOF and NA. For several reasons, including practical wafer flatness and scanning stage errors, a large depth of focus is on the order of 1.0 micrometers is desirable.
There is a rapid loss in DOF as the NA is increased. It is preferable to use shorter wavelengths combined with a low NA to maximize resolution and available DOF simultaneously. A state-of-the-art extreme ultraviolet (EUV) projection system operating at 13.4 nm achieves a 100 nm resolution (assuming K1=0.7) with a depth of focus of 1.0 xcexcm at a NA of 0.10. This large depth of focus improves process latitude, thus enhancing the xe2x80x9cprocess windowxe2x80x9d. In contrast, a deep ultraviolet (DUV) wavelength projection lithography optical system operating at a wavelength (xcex) of 193 nm can only achieve a minimum critical dimension of 180 nm at a numerical aperture of 0.75, assuming the same value for the process dependent constant K1. Further, the depth of focus of the DUV optical system is reduced to 0.34 xcexcm, resulting in a loss of process latitude that adversely impacts device yield. As the process window shrinks, it becomes more difficult to maintain the CD control required for high-density integrated circuits in commercial production.
To produce integrated circuits with ever smaller critical dimensions and higher device density with sufficient process latitude for volume manufacture, it is necessary to develop projection systems operating at extreme ultraviolet (EUV) wavelengths (from 4 to 20 nm). Radiation at these wavelengths must be focused using mirrors coated with multilayer coatings that have high reflectivity at near normal incidence angles. The reflection of radiation off of a mirror is known as xe2x80x9cbouncexe2x80x9d.
State of the art EUV imaging systems have relatively low numerical apertures in the range of 0.08 to 0.10 and can resolve features on the order of 100 nm. To extend the resolution of EUV lithography below 100 nm, optical systems having higher numerical apertures are needed. Increasing the numerical aperture of current EUV designs results in a substantial degradation in the residual wavefront error, making these designs unsuitable for projection lithography.
Prior art contains few examples of high ( greater than 0.10) numerical aperture designs suitable for EUV lithography. An optical system that is usable in the extreme ultraviolet portion of the spectrum is disclosed in U.S. Pat. No. 5,315,629 to Jewell et al., which is herein incorporated by reference. The ""629 patent discloses a four mirror design with a numerical aperture of 0.10 and a ring field width of 0.5 mm. The design has diffraction-limited performance with approximately 10 nanometers of static distortion at the edge of its 0.5 mm ring field. The disclosure states that the numerical aperture of the optical system can be increased to approximately 0.14 without loss in image quality, if the image distortion tolerance is relaxed. The increase in numerical aperture would enable the design to resolve features less than 100 nanometers. However, as this design is re-optimized to minimize the residual wavefront error, the ability to correct distortion over any meaningful ring field width is lost. For example, the residual distortion at a numerical aperture of 0.14 is on the order of 30-40 nm at the edge of the 0.5 mm ring field. This is too much distortion for a practical lithographic projection, even when the effects of scan-averaging are included.
An optical system that is usable in the extreme ultraviolet portion of the spectrum is disclosed in U.S. Pat. No. 5,212,588 to Viswanathan et al., which is herein incorporated by reference. The ""588 patent demonstrate a multi-bounce projection system that incorporates two coaxial aspheric mirrors in a 4-bounce arrangement, where mirror M1 is convex and mirror M2 is concave. To obtain high resolution imagery, the field curvature needs to be corrected to substantially zero so that the imaging surface is perfectly flat. The ""588 projection system is designed so that the two mirrors have substantially the same radius of curvature. Since M1 is convex and M2 is concave, the flat field condition is automatically satisfied (the Petzval sum is made almost identically zero). While the ""588 patent describes a number of embodiments with excellent performance, all the designs suffer one disadvantage in that the exit pupil is centrally obscured. This central obscuration is undesirable since it will degrade resolution and reduce the process latitude for the variety of complex geometries that must be patterned.
In view of the foregoing, there is a need for an optimized optical system that is compatible with short wavelength radiation and has a high numerical aperture for improved resolution, and which addresses the above and other problems.
The present invention is directed to a catoptric optical system that is used to project a reduced mask image onto a wafer with short wavelength radiation. The preferred embodiment comprises an optical system having four mirrored surfaces in a novel five bounce configuration. The imaging bundle is reflected off of one of the mirrored surfaces, mirror M2, twice. The present invention allows for higher device density because the optical system has improved resolution that results from the relatively high numerical aperture. The optical system is designed to have a numerical aperture of approximately 0.15 and operate at a wavelength of approximately 13.1 nm. Under these conditions, resolution on the order of 50 nm can be achieved.
An embodiment of the present invention also includes a well-defined accessible aperture stop that helps to ensure that the imagery is stationary. Illuminated properly, this projection system should have no variation in critical dimension across the field due to vignetting or clipping of the imaging bundles.
Further, an embodiment of the present invention has a balanced static centroid distortion curve across the ring field width. More specifically, the centroid distortion levels at the edges of the ring field width are substantially equal and quantitatively higher than the centroid distortion at the center of the ring field width. By balancing the static centroid distortion curve across the ring field width, the dynamic scan-averaged distortion is minimized.
The mirrors are arranged so that the negative contribution to the Petzval sum from the two reflections off mirror M2 is corrected by the single bounce from mirror M3 and the single bounce from mirror M4. The remaining net negative sum is corrected using mirror M1, which has comparatively little optical power. To minimize the Petzval sum, the radii of mirrors M2, M3, and M4 are approximately equal. The spacing between the mirrors is set so that the imaging bundles are unobscured at the chosen reduction ratio. To minimize the residual wavefront error, all four reflective surfaces of the inventive optical system are aspheric.
Other advantages and features of the present invention will become apparent from a reading of the following description when considered in conjunction with the accompanying drawings of which the following is a brief description.